Warm-blooded animals are subject to attack by parasites, and man has long sought to combat such parasites afflicting domestic companion animals, farmed livestock and exotic animals, to alleviate suffering and for commercial gain. The manner of attack by the parasites, and the identification of a sensitive stage in the life cycle of the parasite, may influence greatly the choice of combating agent. Thus percutaneous treatments using topically applied preparations such as lotions, paints, creams, gels, dusting powders, “pour-ons” and dips are commonly suitable for ectoparasites, but combating endoparasites requires careful selection of the method of administration and the delivery system. Oral drenches, pastes, boluses, tablets, and granules for incorporating into feed mixes are known methods capable of being used by the animal husbandrymen, but other methods which are intended to avoid use of the gastrointestinal route are typically administered by qualified practitioners. Such other methods include use of aerosols, and parenteral drug compositions which are selectively prepared as solution or suspension or micronised powder formulations intended for subcutaneous, intracutaneous, and intramuscular injection according to the intended delivery regime. These last methods require special care in formulation to avoid irritation at the site of injection or possible adverse allergic or pyrogenic reactions.
Injectable formulations are typically prepared using aqueous or non-aqueous (“solvent”) vehicles. The latter class may comprise physiologically tolerable alcohols, glycols, esters, a limited range of organic aromatic solvents, and vegetable oils and extracts or modified forms thereof. In selecting vehicles, the skilled worker has to consider a number of issues including, solubility of the intended active ingredient(s), the affinity of the drug to certain vehicles, whether it will affect any essential auxiliaries, pH, stability over time, viscosity, and naturally the risk of any toxic effect upon the animal to be treated. Therefore, formulation of a parasiticide is a complex task.
Traditional parasiticides include chemical agents such as the benzimidazoles, and carbamates, and plant extracts such as the pyrethroids, which tend to be used to combat ectoparasites such as ticks and mites.
The avermectins are very potent antiparasitic agents which are useful against a broad spectrum of endoparasites and ectoparasites in mammals as well as having agricultural uses against various parasites found in and on crops and soil. The basic avermectin compounds are isolated from the fermentation broth of the soil micro-organism Streptomyces avermitilis and these compounds are described in U.S. Pat. No. 4,310,519. Furthermore, derivatives of these basic avermectin compounds have been prepared by a variety of chemical means.
Some of the avermectin group of compounds contain a 22, 23-double bond and others contain a disaccharide at the 13-position which consists of α-L-oleandrosyl-α-L-oleandrosyl group. One or both saccharide units can be removed forming a monosaccharide or an aglycone (where both saccharides are removed) as described in U.S. Pat. No. 4,206,205. The aglycone derivatives possess a hydroxy group at the 13 position which may be removed to form the 13-deoxy compound as described in the U.S. Pat. No. 4,171,314 and U.S. Pat. No. 4,173,571. Acylation of hydroxy groups on the avermectin compounds and derivatives can be carried out as described in U.S. Pat. No. 4,201,861.
The milbemycin series of compounds, disclosed in U.S. Pat. No. 3,950,360, are structurally similar to the avermectin family in that they contain the sixteen membered macrocyclic ring. However, they do not contain the disaccharide sub-unit and there are differences in the substituent groups.
Ivermectin, disclosed in U.S. Pat. No. 4,199,569, is prepared by the selective reduction of the 22, 23 double bond of the avermectin compounds. Ivermectin is a mixture of 22, 23-dihydro Avermectin B1a and B1b in a ratio of at least 80:20.
Ivermectin is an especially preferred active component in pesticidal compositions, and there is extensive literature on its activity, demonstrating its efficacy against internal and external parasites, and its ability to interfere in the life cycle of certain parasites. The Merck Index (1996) cites several references including J. C. Chabala et al, J. Med. Chem. 23, 1134 (1980); J. R. Egerton et al, Brit. Vet. J. 136, 88 (1980); W. C. Campbell et al, Science 221, 823-828 (1983) to mention but a few.
Formulation of ivermectin for the purposes of delivery in a variety of presentations, e.g. as an oral drench, pour-on, parenteral formulations, granules for adding to feed, and syringeable pastes. has proved highly challenging and numerous patents have been published on its use. Ivermectin exhibits a lipophilic character but it can be solvated in aqueous systems, and various patents describe special solvent systems for use in its formulation. Thus reference may be made at least to EP 0 045 655, and EP 0 146 414 for example.
Although ivermectin is surprisingly effective, and has enjoyed a long period of commercial success, there remains a keen interest in exploiting ivermectin against a wider range of parasites and in overcoming tolerance by some parasites which demands higher amounts of ivermectin to be delivered. Taking into account the fact that a significant volume of use of ivermectin is in protecting and treating animals intended for human consumption, there are constraints on the residual amount of ivermectin in the carcass of such an animal. Therefore, high loadings of ivermectin, even if technically feasible, in a delivery system are not necessarily the optimum solution.
Combination formulations are also desirable taking account of acquired tolerance or resistance in pests to prolonged usage of other more traditional parasiticidal agents. This phenomenon is well documented, e.g. in relation to worming compositions. Synergistic effects or complementary effects of combined parasiticidal agents have been observed as a route to combating the aforesaid tolerance problem. Synergistic anthelmintic compositions are discussed in WO 94/28887, which focuses on substituted mono- and bisphenols, salicylanilides, benzene sulphonamides, halogenated benzimidazoles, benzimidazoles, and benzimidazole carbamates.
The salicylanilides, tend to be effective against fungal attack, but the chemically modified derivative closantel is an effective worming agent. Closantel is described in U.S. Pat. No. 4,005,218 and in the literature, e.g. J. Guerrero et al, J. Parasitol. 68, 616, (1983); H. Van den Bossche et al, Arch. Int. Physiol. Biochim, 87, 851(1979); H. J. Kane et al, Mol. Biochem. Parasitol. 1, 347(1980).
The opportunity to combine the use of avermectins with other parasiticidal agents has been explored already. Thus one finds that skin-absorbable pour-on formulations containing triclabendazole, optionally containing an avermectin, tetramisole or levamisole have been proposed in WO 0061068. An injectable formulation containing closantel together with an avermectin or milbemycin has been proposed in WO 95/05812. Formulations of the pour-on and injectable type are discussed in WO 01/60380, which comprise use of a pyrrolidone solvent and a bridging solvent such as a xylene, optionally including a further solubility agent such as propylene glycol caprylic acids and esters or peanut oil. This special solvent system is needed to address the difficulties of formulating differing parasiticidal agents such as closantel and ivermectin together.
Salicylanilide derivatives such as closantel provide useful control over a range of parasites and are especially useful against liver fluke. The avermectin group of anti-parasitic compounds of which ivermectin is the best known example, provide complementary protection against many other parasites such as roundworms. Therefore, there are advantages to be gained if a combination of these drugs could be provided in a form that can be conveniently administered to livestock and which will provide effective control of parasitic infection.
For ivermectin and closantel the established dose rate for injection of livestock is of the order of 200 μg/kg (ivermectin) and 2.5 mg/kg (closantel). Provision of a satisfactory aqueous formulation is problematical because the optimum pH for each drug is different. An acidic system providing the optimum pH for ivermectin, whereas closantel requires an alkaline medium for satisfactory dissolution.
Accordingly non-aqueous or essentially non-aqueous formulations were investigated. Ivermectin can be prepared in non-aqueous or low water content systems that are suitable for injection. Glycerol formal, propylene glycol, polyethylene glycol, pyrrolidone, and related solvents have been used in various formulations, singly or in combination. Patent publication WO 95/05812 discloses closantel and ivermectin solutions for injection using some of the solvents established as suitable for ivermectin (glycerol formal, polyethylene glycol, propylene glycol and water). However the effectiveness of the formulations, in terms of bioavailability of the active parasiticidal agents, described in that patent application were not disclosed.
The results of our research into the efficacy of such formulations are summarised in Table 1 presented hereinbelow.
In order to evaluate formulations of the type described in the reference WO 95/05812, administration of an ivermectin/closantel combination product as disclosed therein, at a dosage corresponding to 2.5 mg/kg closantel was carried out according to established industry practices. However, this failed to produce blood plasma levels of closantel greater than 20 ppm (Examples 1 and 2 in Table 1 reported hereinafter). According to typical experience, it was anticipated that a higher amount of closantel would have a favourable effect on the blood plasma levels. Increasing the closantel concentration in such formulations was readily achieved, allowing higher dosing of closantel in the combination product. Despite these attempts at higher dosing, administration of closantel at 5 mg/kg did not increase blood plasma levels at all and even at the exceptionally high dosing level of 7.5 mg/kg the blood plasma levels only increased to 31 ppm (Examples 3 and 4 in Table 1). Therefore the proposed formulations available from following the teachings of the reference WO 95/05812 surprisingly failed to deliver the expected solution to the problem of obtaining a satisfactory combination product.